The present invention relates to a process for filtering an electrical signal by the transfer of charges into a semiconductor. It also relates to switched capacitor filters using such a process.
Active filters called switched capacitor filters are known, particularly from two articles in the American "IEEE Journal of solid-state circuits", vol. SC-12, No. 6, December 1977, pp. 592 to 608.
Switched capacitor filters generally have at least one amplifier associated with a network of MOS switches and capacitors, in which the combination of two MOS switches and one capacitor serves as the resistor. Switched capacitor filters have small overall dimensions, as well as high linearity and temperature stability.
The capacitors of switched capacitor filters are integrated in per se known manner by a procedure involving two metallisation levels. Each terminal of the integrated capacitors then has a stray capacitance of non-negligible value in relation to the substrate. The stray capacitances created by the capacitors of the network associated with the amplifier are particularly disadvantageous, because they lead to errors in the frequency response of the filter. It is then necessary to increase the values of the capacitors of the system in order to reduce the effect of stray capacitances, which increases the surface of the semiconductor substrate necessary for producing the filter, and therefore its cost and overall dimensions.